1. Field of the Invention
The present invention broadly relates to methods for producing shaped metal matrix composites. More particularly, this invention relates to methods for producing shaped metal matrix composites by infiltrating spontaneously a permeable mass of ceramic filler with molten aluminum alloy until the latter contacts a barrier means for establishing a surface, perimeter, boundary or the like, in order to produce net shapes.
2. Description of the Prior Art
Composite products comprising a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like, show great promise for a variety of applications because they combine the strength and hardness of the strengthening phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite will show an improvement in such properties as strength, stiffness, contact wear resistance, and elevated temperature strength retention relative to the matrix metal, per se, but the degree to which any given property may be improved depends largely on the specific constituents, their volume or weight fraction, and how they are processed in forming the composite. In some instances, the composite also may be lighter in weight. Aluminum matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their higher stiffness, wear resistance and high temperature strength relative to aluminum.
Various metallurgical processes have been described for the fabrication of aluminum matrix composites, ranging from methods based on powder metallurgy techniques to those involving liquid-metal infiltration such as by pressure casting, and the use of vacuum or wetting agents to effect infiltration of metal into a ceramic mass.
A method for producing a metal matrix composite by the spontaneous infiltration of a permeable mass of ceramic filler material with a molten metal is disclosed in the commonly owned and copending U.S. Pat. No. 4,828,008, which issued on May 9, 1989, and was entitled "Metal Matrix Composites" and in the name of Danny Ray White et al. In accordance with the method of this invention, a metal matrix composite is produced by infiltrating a permeable mass of ceramic filler or ceramic coated filler with molten aluminum containing at least about 1% by weight magnesium, and preferably at least about 3% by weight. Infiltration occurs spontaneously without the need of external pressure or high vacuum. A supply of the molten metal alloy is contacted with the mass of filler material at a temperature of at least about 700.degree. C. in the presence of a gas comprising from about 10 to 100%, and preferably at least about 50%, nitrogen by volume, balance nonoxidizing gas, e.g. argon. Under these conditions, the molten aluminum alloy infiltrates the ceramic mass under normal atmospheric pressures to form an aluminum matrix composite. When the desired amount of ceramic material has been infiltrated with molten alloy, the temperature is lowered to solidify the alloy, thereby forming a solid metal matrix structure that embeds the reinforcing ceramic material. Usually, and preferably, the supply of molten alloy delivered will be sufficient to allow the infiltration to proceed essentially to the boundaries of the ceramic mass. The entire disclosure of this commonly owned patent is expressly incorporated herein by reference.
An element in using the prior art methods and the methods of the above-mentioned commonly owned patent to produce a net or near net shape metal matrix composite is to minimize or inhibit transport or infiltration of the metal matrix beyond the defined surface boundaries of the permeable mass of ceramic filler or preform. Overinfiltration of the surface boundaries can be substantially prevented by controlling the infiltration of the metal matrix to any defined surface boundaries, which may be accomplished such as by using a predetermined quantity of metal, exhausting the atmosphere of nitrogen-containing gas, or lowering the temperature below the metal's melting point. Any of these steps may require close control or vigilance to obtain essentially no transport of the metal matrix beyond the defined surface boundary of a mass of ceramic filler or preform, and still may not produce the most desirable net or near net shape, or may require additional machining or finishing.
The present invention provides means for producing a metal matrix composite under atmospheric pressure while reliably establishing a boundary or substantially preventing overinfiltration or transport beyond the boundary of the infiltrating metal matrix, which is desirable in forming net shapes in metal matrix composites.